Effect of interfaces on gas breaktrough pressure in compacted bentonite used as engineered barrier for radioactive waste disposal

In a deep geological nuclear waste repository gas can be generated by different processes. Understanding the gas transport mechanisms across the engineered and natural barriers in a repository is relevant for its security assessment, both in terms of mechanical stability and of radionuclide transport. The engineered barrier may be composed of compacted blocks of bentonite and the interfaces between these blocks might evolve into preferential fluid pathways, in particular for the gas generated around the waste canisters. Small-scale laboratory tests were performed in sound samples and in samples crossed by an interface to determine gas breakthrough pressure values after saturation and the effect on them of the interface. The FEBEX bentonite, a Spanish bentonite composed mainly of montmorillonite, was used in the tests. The gas breakthrough pressure of the saturated compacted samples increased with dry density and was higher than the swelling pressure of the bentonite. Gas breakthrough could take place either in an instantaneous or in a gradual way, the difference between both modes being the flow rate, much higher in the first case. The gas transport mechanism would be microscopic pathway dilation, with microfracturing in the case of the instantaneous episodes. A sealed interface along the bentonite did not seem to affect the breakthrough pressure or gas permeability values, since the behaviour patterns were similar in both kinds of samples, depending mostly on the bentonite dry density.